App in Scala

application里一个object extend了一个App:

通过查看source code的实现能发现App是一个trait，继承了DelayedInit:

scala.DelayedInit

Classes and objects (but note, not traits) inheriting the DelayedInit marker trait will have their initialization code rewritten as follows: code becomes delayedInit(code).

Initialization code comprises all statements and all value definitions that are executed during initialization.

Example:

trait Helper extends DelayedInit { def delayedInit(body: => Unit) = { println("dummy text, printed before initialization of C") body // evaluates the initialization code of C } } class C extends Helper { println("this is the initialization code of C") } object Test extends App { val c = new C }

Should result in the following being printed:

dummy text, printed before initialization of C this is the initialization code of C

所有带有App 特质的类，其初始化方法都会被挪到delayedInit方法中。App特质的main方法捕获到命令行参数，调用delayedInit方法。